1. Field of the Invention
The present invention relates generally to temperature responsive ceramic coatings for application to the surface of a substrate. At ambient temperatures, each individual coating exhibits a distinctive color and texture. When the temperature of the substrate on which the coating is applied is raised to, or above, a predetermined transition temperature, visual and physical characteristics of the coating change in a distinct manner, thereby indicating that the predetermined transition temperature has been reached.
2. Description of the Prior Art
There are numerous known techniques for measuring the temperature of a surface. For example, thermocouples have long been used for this purpose. However they are objectionable for certain applications because they occupy substantial space, are relatively fragile, require long leads subject to entanglement, and cannot withstand excessive temperatures for extended periods of time.
A modern form of an old temperature measuring concept is disclosed in U.S. Pat. No. 3,724,272 directed toward a fusible pyrometric device for measuring the amount of heat applied during a heat treatment process, such as the firing of ceramic ware in a kiln.
Temperature sensing of a substrate such as an electronic circuit is oftentimes desirable. U.S. Pat. No. 3,874,240 accomplishes such sensing by dispersing in a carrier material such as glass a thermochromic composition comprising particles of an electron-donor compound weakly associated with particles of an electron-acceptor compound. The carrier material is suitably attached to the substrate and seals the compounds from the effects of the environment while permitting the compounds to weakly associate at a first temperature to form a first indicative color and disassociate at a lower temperature to form a second indicative color.
A variety of temperature measuring devices have also been devised which employ fiber optic arrangements for illuminating and detecting an object whose radiance is a measure of its temperature. Such objects may be in relatively inaccessible locations and examples of such temperature measuring devices are disclosed in U.S. Pat. Nos. 4,278,349; 4,307,607; 4,576,486; 4,673,299; and 4,689,483.
Also of interest in providing a complete background of developments which have lead to the present invention are U.S. Pat. Nos. 2,739,909; and 3,377,165. In the former instance, there is a disclosure of a thermo-sensitive paper with a substantially opaque organic coating on its outer surface which softens under a heated stylus and becomes translucent at the region scribed by the stylus to expose a substrate material. The latter patent concerns a somewhat similar concept, and relates to a light sensitive copy sheet which comprises a supporting substrate or carrier and an initially opaque and porous organic binder layer. On exposure to a light-image and to a moderately elevated temperature, the organic binder layer is selectively transparentized at the light-struck areas, thereby forming a image.
Temperature sensing patches which change color according to temperature are disclosed in U.S. Pat. No. 4,459,046. In this instance, using a pressure sensitive adhesive coating, the patches are applied to a person's skin or to some other supporting surface whose temperature is to be measured. Similar devices which utilize incapsulated liquid crystals are disclosed in U.S. Pat. Nos. 3,661,142; 4,302,971; and 4,448,204.
Thermal indicators of the type wherein an organic coating is applied to a base and either reversibly or irreversibly changes its appearance at a predetermined elevated temperature due to physical changes occurring therein at such temperatures have long been known. U.S. Pat. Nos. 2,261,473; 2,269,038; 2,809,116; 2,928,791; 4,301,054; and 4,344,909 are representative of this form of indicator.
Such indicators which are sometimes referred to as "thermal paints" employ mixtures of organic dyes or pigments to effect changes of color over a wide range of temperatures. When the temperature of a treated surface is raised, the original color of the pigment changes sharply at a definite point and the new color persists after the surface has cooled down. As in the instance of the present invention, thermal paints are of value in those instances in which a change of temperature only is desired to be ascertained, usually at some critical point or points, the exact temperature over a considerable range not being of importance. Unfortunately, in general, thermal paints possess a number of drawbacks which the present invention seeks to avoid. Specifically, in many instances, thermal paints often adhere poorly to a substrate, are toxic, and require use of costly materials. They also may be reversible, sometimes being subject to chemical change depending upon the particular gases present in the environment in which they are required to operate. As an added drawback, thermal paints do not customarily exhibit a texture change simultaneously with a change of color. Furthermore, the longer that thermal paints are subjected to a given elevated temperature after lapse of an initial period of time customary to assure substantial temperature uniformity throughout the substrate, the lower the transition temperature at which the color change occurs. Indeed, with thermal paints, this rate of lowering of the transition temperature is quite rapid and is aggravated at higher temperatures.
A particularly pertinent prior art reference is U.S. Pat. No. 3,415,122 which discloses a temperature indicator in the form of a ceramic film for sensing and recording temperatures in the range of 900.degree. C.-1400.degree. C. The film may be attached, for example, to the outer skin of a space vehicle. When recovered after re-entry of the space vehicle, the indicator is cleaned, polished, and photomicrographed, then compared with calibration photomicrographs to determine the maximum temperature to which the space vehicle was subjected. Of course, it will be appreciated that such a procedure is extremely time consuming and labor intensive and, therefore, costly, nor are results readily available.